This application is related to applicant""s co-pending application Ser. No. 09/548,388, filed on Apr. 11, 2000, commonly assigned with the present invention. The field of this invention is high speed 0.8 micron (xcexcm) infrared photodetectors. Its purpose is to achieve an increased high frequency response of the high electron mobility transistor photodetector (HEMT-PH) without any degradation in the responsivity. Known HEMT-PH devices exhibit a relatively low maximum response frequency when used in the photodetector mode compared to the maximum response frequency when the device is used as a high electron mobility transistor (HEMT). The relatively low maximum response frequency of the HEMT-PD has been attributed to slow drift velocity of photoexcited minority holes which are present in the HEMT-PD buffer layer. For background, see, S. Banba, E. Suematsu, and H. Ogawa, Proc. 22 European Microwave Conf., 747 (1992), incorporated herein by reference as if fully set forth. Devices according to the present invention solve the problem of slow drift velocity of photoexcited minority holes in the HEMT-PD buffer layer thus resulting in a faster, more sensitive HEMT-PD than has been achieved in the past. HEMT-PDs of the present application will exhibit much higher photocurrent gain than photoconductive detectors due to the combination of photoconductive and photovoltaic effects on the photcurrent in the HEMT-PD channel as well as the high electron mobility of the HEMT-PD due to the use of modulation doping in the HEMT-PD heterostructure. Modulation doping in the HEMT-PD results in a higher channel electron mobility, higher saturated channel current and a shorter electron transit time; which combine to giver a greater photocurrent gain than the photoconductive detector.
GaAs grown at low temperatures (LT-GaAs), i.e., GaAs layers grown by molecular beam epitaxy at substrate temperatures of 250-300xc2x0 C., is known to exhibit very short carrier lifetimes ( less than 1ps) and semi-insulating behavior after annealing. Therefore, this material is may be used to increase the capture rate of photogenerated holes in the GaAs photo absorption region resulting in a faster removal of holes from the absorption region and an increased speed HEMT-PD. A disadvantage of LT GaAs is the higher cost of fabrication.
HEMT-PDs according to the present invention have many possible applications. For example, they can be used as a 0.8 micron photodetector in optoelectronic integrated circuit receivers which are used in short wavelength fiber optic subcarrier transmission links in personal communication systems. They can also be used in optical local area networks, optical signal processors, distributed fiber optic sensors, cable television networks and satellite communication systems. The devices are compatible with monolithic microwave integrated circuits.
The invention of this application increases the HEMT-PD""s high frequency response and sensitivity by using an LT GaAs layer to increase the capture rate of photoexcited holes and a negatively biased substrate electrode to increase the drift velocity of the photoexcited holes resulting in a faster removal of holes from the photo-absorption region. While the use of a substrate bias voltage was first proposed to decrease the fall time of a simple photoconductive detector, it has not been applied to a HEMT-PD. For background, see, C. Y. Chen, Y. Pang, A. Cho and P. Garbinski, Appl. Phys. Lett. 43, 1115 (1983).
The principal object of the invention is to achieve an increase in the maximum response frequency of the detector. Another object is to achieve a reduction in the high frequency device noise. These and other objects are achieved, at least in part, by a high electron mobility transistor photodetector which includes a GaAs undoped substrate, a p-type GaAs layer positioned above the GaAs substrate that is between 0.5 to 1 xcexcm in thickness, an undoped low temperature GaAs layer positioned above the p-type GaAs layer, an undoped GaAs layer buffer positioned above the undoped low temperature GaAs (LT GaAs) layer, a layer of undoped InGaAs positioned above the undoped GaAs layer, a layer of undoped AlGaAs positioned above the layer of InGaAs, an n+ AlGaAs charge-supplying layer positioned above the layer of undoped AlGaAs, an n+ GaAs contact layer positioned above the n+ AlGaAs charge-supplying layer, and source and drain ohmic contacts positioned above the n+ GaAs contact layer. In a preferred embodiment, the high electron mobility transistor photodetector also includes an ohmic contract for supplying a negative bias voltage to the p-type GaAs layer.